1. Field of the Invention
The present invention generally relates to a method for manufacturing LED devices, and more particularly to a method for manufacturing GaN LED devices by using aligned wafer bonding technology.
2. Description of the Prior Art
Since GaN possesses a wide band gap (Eg=3.4 eV in room temperature) and its spectrum is near the wavelength of blue light, GaN is suitable for manufacturing short wavelength LED devices, and it further becomes one of the most popular materials for developing photoelectric elements. Nowadays, GaN can grow steadily on a sapphire substrate. Moreover, GaN can be utilized in making short wavelength light-emitting devices. However, because the sapphire has low thermal conductivity, it will worsen the reliability of the device.
FIGS. 1A to FIG. 1D are cross-sectional diagrams of a traditional method for manufacturing LED devices. The details are disclosed in U.S. Pat. No. 6884646 entitled ‘METHOD FOR FORMING AN LED DEVICE WITH A METALLIC SUBSTRATE.’ First, a provisional substrate 20 is provided. Then, a LED epitaxial layer 21, a first electrode layer 25, and a conduction enhancing layer 26 are sequentially formed on the provisional substrate 20. Next, a metallic permanent substrate 27 is formed on the conduction enhancing layer 26. Next, the provisional substrate 20 is removed to expose a surface of the LED epitaxial layer 21. Then, a plurality of second electrodes 28 are formed on the surface of the LED epitaxial layer 21. Finally, the metallic permanent substrate 27, the conductive enhancing layer 26, the first electrode layer 25 and the LED epitaxial layer 21 are cut to form a plurality of LED devices. The provisional substrate 20 can be sapphire. Because sapphire has low thermal conductivity, the provisional substrate 20, i.e. sapphire, is removed after a metallic permanent substrate 27 is formed on the GaN LED epitaxial layer 21. Accordingly, the LED devices have good thermal conductivity, and good electrostatic discharge effect, and therefore are capable of operating on high current condition. However, by electroplating technology the metallic permanent substrate 27 is formed. Accordingly, the difference of coefficient of thermal expansion between the metallic permanent substrate 27 and the semiconductor will cause the metallic permanent substrate 27 bending or peeling.
In view of the abovementioned drawbacks of the traditional method for manufacturing LED devices, there is a need for a new method for manufacturing LED devices, whose permanent substrates not only have good thermal conductivity, i.e. good heat dissipation ability and good electrostatic discharge effect, but also are free from bending or peeling.